This invention relates to the manufacture of low density paper products.
The paper manufacturing process is very old. In recent years there has been an increasing demand for printing papers having excellent physical properties. On the other hand, there is a great demand for weight reduction in these papers for the sake of reduced cost in transportation and mailing. These demands historically have been mutually contradictory, given that higher quality papers conventionally have a higher base paper basis weight, and higher coat weight if a coating is applied. A paper with a lower basis weight may be selected in order to reduce the weight of a paper article made therefrom, but that is not an ideal solution since it will result in thinner paper and diminish the feeling of bulk expected from a paper product. It is further desired when decreasing the basis weight of a paper to maintain the stiffness of the paper at a lower sheet thickness. For these reasons, the market is presently demanding high quality paper articles that offer greater paper thickness at a given basis weight or a lower basis weight at a given paper thickness.
In the course of manufacturing paper and similar products, such as paper board, it is well known to incorporate inorganic fillers into the fibrous web in order to improve the quality of the resulting product. The fillers are important in improving the printing qualities of the paper by improving the surface characteristics, and the use of an appropriate filler vastly improves the opacity and brightness of a paper sheet. A number of inorganic materials have long been known to be effective for this purpose but despite the effectiveness of these inorganic fillers lower density replacements have been much sought after.
Modern paper manufacturers are constantly searching for a way to obtain paper having lower density while maintaining desired mechanical properties, thermal insulation and optical properties. Various approaches have been tried, including the use of various organic and inorganic materials as fillers.
The use of polymeric microspheres as a filler for paperboard is disclosed in U.S. Pat. No. 6,379,497 B1. U.S. 2002/014632 A1 discloses the use of expandable microspheres in the manufacture of opaque tissue paper. U.S. 2001/0038893 A1 teaches that expanded microspheres can be used in the manufacture of a low density paperboard material having insulating properties. Japanese published patent applications 2000-053351, 01-210054 and 11-006466 disclose the use of a hollow polymer particle in the wet-end of the paper-making where the hollow particle is cationic in nature. Japanese published patent application 2000-160496 teaches the use in the wet-end of the paper-making process of a composite hollow particle obtained by the adsorbtion of a high molecular weight amphoteric polyelectrolyte onto the surface of a hollow particle. However, the amount of treated composite hollow particle that is retained in the paper is to low to be practical. An additive to the paper manufacture process must be retained by the sheet in order to function properly. U.S. Pat. No. 6,139,961 discloses the incorporation of hollow sphere organic pigment into the formed wet sheet for improving the strength and opacity of the paper.
The problem of low cost manufacturing of a paper product simultaneously having high bulk and enhanced optical properties while maintaining acceptable mechanical properties has not been solved in the prior art.